The One Sith encompasses both the practitioners of the Sith religion and their other associates such as Sith soldiers and agents.
The Sith are an order of Force Sensitives who utilize the darkside of the force. The Sith are the most infamous of all dark side practitioners and known to dedicate their whole lives to it. Multiple empires have risen and fallen under the reign of the sith, and with such great influence the sith religion inspired many cults that weren't technically a part of the Sith order, nor made up of actual Sith.
The One Sith follow a single dark lord, the embodiment of the darkside to them. While the one sith are noted to frequently have fanatically loyal members, it is not without its occasional infighting in bids for power. The core theory behind the rule of one is to focus the sith as a whole toward one leader and one purpose to better utilize their assets and numbers without losing the harsh training and environment that creates true sith.
Following the death of Darth Krayt, the One Sith survived the loss of its founder and planned to continue Krayt's plans to subjugate the galaxy under Sith dominance, but decided on going about it in a different approach. Thus, the Sith intended to infiltrate every planetary government with the main objective of destroying the new galactic regime from within.
The Return of the Galactic Empire
The battle for the inner worlds began long before the Sith arrived. Years ago, all levels of the government from the Senate to the SIS to the Defense Forces were infiltrated. While the Jedi Order may not have been infiltrated, Darth Hex has claimed to have projected false visions through the force to cause paranoia and misdirection amongst some of the order, perhaps leading to some Jedi Knights abondoning the Order in the darkest days. The Empire seems to have achieved its goal of taking the capital by destabilizing the Republic and Jedi Order.
Battle of Bastion
With the the Republic forces spread thin and the galaxy plunged into chaos, the Sith continued their conquest of Remnant space, ever increasing their forces. After months of war, the Sith army finally ended its campaign with the fall of Bastion.
In Bastion's surrounding space, the Fel Sovereignty engaged the Sith Starbreaker with their own Super Star Destroyer, the Sovereignty. During the large scale battle the Starbreaker rammed the Sovereignty, breaching the hull and allowing Sith forces to board and take possession of it.
While the remaining planetary defenses were dismantled, Sith forces invaded the heavily fortified planet, fighting in a month long battle across the surface. Eventually the Sith managed to reach the Fel Palace where the Dark Lord of the Sith, Darth Indal, struck down the Fel Emperor, ceasing control over the planet.This heralded the return of the Galactic Empire, led by the new Emperor, Darth Indal.
Fall of Coruscant
With the Republic crumbling, their forces had no other choice but to withdraw as the new Emperor targeted the Core worlds. Confronting the Galactic City Authority with force, Coruscant was surrendered allowing the Sith to now occupy the planet and most of the Galactic Core.
The initiates of the darkside. Disposable and numerous, many die before they ever reach the chance to take their trials for advancement. Acolytes spend the majority of their time absorbing as many lessons from the masters as they can, they are sent on minor tasks sometimes to test their abilities in often what amounts to dangerous situations. The end goal of an Acolyte is to be chosen by a higher ranking Sith to serve as a full time apprentice; though some who are not so successful in gaining the attention of a master end up working primarily with soldiers.
As with any larger force, regular soldiers are in demand. These are those who have been conscripted, hired, or bargained with to join the sith's agenda with more traditional armaments then these strange wizardly powers and glowing sticks. Sith soldiers are highly disciplined and expected to observe a proper military behavior unlike some mercenaries. While they are not technically 'sith' general misconceptions have resulted in them being called sith troopers by affiliation.
Experts in espionage and diplomatic relations. The agents are tasked to managing the sith relations with other groups, whether that group knows it or not. Whenever a problem needs discretion, agents are sent. The position is open to both sith and regular folk gifted in these particular arts.
Possesses a deep understanding of force powers and using them to their point of limitation.
Creates and manipulates objects, life, or power to create a desired effect.
Utilizes items and spells to enhance their powers that they could not otherwise generate by their own means. Lacks an understanding of how the enhancements work.
The backbone of the order, the Marauder is the front line assault, charging in with strength and power.
Values stealth, agility, avoidance and speed over front line combat.